DeltaContext.h

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#ifndef ZSERIO_DELTA_CONTEXT_H_INC
#define ZSERIO_DELTA_CONTEXT_H_INC
 
#include <type_traits>
 
#include "zserio/BitStreamReader.h"
#include "zserio/BitStreamWriter.h"
#include "zserio/RebindAlloc.h"
#include "zserio/Traits.h"
#include "zserio/Types.h"
#include "zserio/UniquePtr.h"
#include "zserio/Vector.h"
 
namespace zserio
{
 
namespace detail
{
 
// calculates bit length on delta provided as an absolute number
inline uint8_t absDeltaBitLength(uint64_t absDelta)
{
    uint8_t result = 0;
    while (absDelta > 0)
    {
        result++;
        absDelta >>= 1U;
    }
 
    return result;
}
 
// calculates bit length, emulates Python bit_length to keep same logic
template <typename T>
uint8_t calcBitLength(T lhs, T rhs)
{
    const uint64_t absDelta = lhs > rhs
            ? static_cast<uint64_t>(lhs) - static_cast<uint64_t>(rhs)
            : static_cast<uint64_t>(rhs) - static_cast<uint64_t>(lhs);
 
    return absDeltaBitLength(absDelta);
}
 
// calculates delta, doesn't check for possible int64_t overflow since it's used only in cases where it's
// already known that overflow cannot occur
template <typename T>
int64_t calcUncheckedDelta(T lhs, uint64_t rhs)
{
    return static_cast<int64_t>(static_cast<uint64_t>(lhs) - rhs);
}
 
} // namespace detail
 
class DeltaContext
{
public:
    DeltaContext() = default;
    ~DeltaContext() = default;
 
    DeltaContext(DeltaContext&& other) = default;
    DeltaContext& operator=(DeltaContext&& other) = default;
    DeltaContext(const DeltaContext& other) = default;
    DeltaContext& operator=(const DeltaContext& other) = default;
    template <typename ARRAY_TRAITS, typename OWNER_TYPE>
    void init(const OWNER_TYPE& owner, typename ARRAY_TRAITS::ElementType element)
    {
        m_numElements++;
        m_unpackedBitSize += bitSizeOfUnpacked<ARRAY_TRAITS>(owner, element);
 
        if (!isFlagSet(INIT_STARTED_FLAG))
        {
            setFlag(INIT_STARTED_FLAG);
            m_previousElement = static_cast<uint64_t>(element);
            m_firstElementBitSize = static_cast<uint8_t>(m_unpackedBitSize);
        }
        else
        {
            if (m_maxBitNumber <= MAX_BIT_NUMBER_LIMIT)
            {
                setFlag(IS_PACKED_FLAG);
                const auto previousElement = static_cast<typename ARRAY_TRAITS::ElementType>(m_previousElement);
                const uint8_t maxBitNumber = detail::calcBitLength(element, previousElement);
                if (maxBitNumber > m_maxBitNumber)
                {
                    m_maxBitNumber = maxBitNumber;
                    if (m_maxBitNumber > MAX_BIT_NUMBER_LIMIT)
                    {
                        resetFlag(IS_PACKED_FLAG);
                    }
                }
                m_previousElement = static_cast<uint64_t>(element);
            }
        }
    }
 
    template <typename ARRAY_TRAITS, typename OWNER_TYPE>
    size_t bitSizeOf(const OWNER_TYPE& owner, typename ARRAY_TRAITS::ElementType element)
    {
        if (!isFlagSet(PROCESSING_STARTED_FLAG))
        {
            setFlag(PROCESSING_STARTED_FLAG);
            finishInit();
 
            return bitSizeOfDescriptor() + bitSizeOfUnpacked<ARRAY_TRAITS>(owner, element);
        }
        else if (!isFlagSet(IS_PACKED_FLAG))
        {
            return bitSizeOfUnpacked<ARRAY_TRAITS>(owner, element);
        }
        else
        {
            return static_cast<size_t>(m_maxBitNumber) + (m_maxBitNumber > 0 ? 1 : 0);
        }
    }
 
    template <typename ARRAY_TRAITS, typename OWNER_TYPE>
    typename ARRAY_TRAITS::ElementType read(const OWNER_TYPE& owner, BitStreamReader& in)
    {
        if (!isFlagSet(PROCESSING_STARTED_FLAG))
        {
            setFlag(PROCESSING_STARTED_FLAG);
            readDescriptor(in);
 
            return readUnpacked<ARRAY_TRAITS>(owner, in);
        }
        else if (!isFlagSet(IS_PACKED_FLAG))
        {
            return readUnpacked<ARRAY_TRAITS>(owner, in);
        }
        else
        {
            if (m_maxBitNumber > 0)
            {
                const int64_t delta = in.readSignedBits64(static_cast<uint8_t>(m_maxBitNumber + 1));
                const typename ARRAY_TRAITS::ElementType element =
                        static_cast<typename ARRAY_TRAITS::ElementType>(
                                m_previousElement + static_cast<uint64_t>(delta));
                m_previousElement = static_cast<uint64_t>(element);
            }
 
            return static_cast<typename ARRAY_TRAITS::ElementType>(m_previousElement);
        }
    }
 
    template <typename ARRAY_TRAITS, typename OWNER_TYPE>
    void write(const OWNER_TYPE& owner, BitStreamWriter& out, typename ARRAY_TRAITS::ElementType element)
    {
        if (!isFlagSet(PROCESSING_STARTED_FLAG))
        {
            setFlag(PROCESSING_STARTED_FLAG);
            finishInit();
            writeDescriptor(out);
 
            writeUnpacked<ARRAY_TRAITS>(owner, out, element);
        }
        else if (!isFlagSet(IS_PACKED_FLAG))
        {
            writeUnpacked<ARRAY_TRAITS>(owner, out, element);
        }
        else
        {
            if (m_maxBitNumber > 0)
            {
                // it's already checked in the init phase that the delta will fit into int64_t
                const int64_t delta = detail::calcUncheckedDelta(element, m_previousElement);
                out.writeSignedBits64(delta, static_cast<uint8_t>(m_maxBitNumber + 1));
                m_previousElement = static_cast<uint64_t>(element);
            }
        }
    }
 
    // overloads with dummy owner
 
    template <typename ARRAY_TRAITS>
    void init(typename ARRAY_TRAITS::ElementType element)
    {
        init<ARRAY_TRAITS>(DummyOwner(), element);
    }
 
    template <typename ARRAY_TRAITS>
    size_t bitSizeOf(typename ARRAY_TRAITS::ElementType element)
    {
        return bitSizeOf<ARRAY_TRAITS>(DummyOwner(), element);
    }
 
    template <typename ARRAY_TRAITS>
    typename ARRAY_TRAITS::ElementType read(BitStreamReader& in)
    {
        return read<ARRAY_TRAITS>(DummyOwner(), in);
    }
 
    template <typename ARRAY_TRAITS>
    void write(BitStreamWriter& out, typename ARRAY_TRAITS::ElementType element)
    {
        write<ARRAY_TRAITS>(DummyOwner(), out, element);
    }
 
private:
    struct DummyOwner
    {};
 
    void finishInit()
    {
        if (isFlagSet(IS_PACKED_FLAG))
        {
            const size_t deltaBitSize = static_cast<size_t>(m_maxBitNumber) + (m_maxBitNumber > 0 ? 1 : 0);
            const size_t packedBitSizeWithDescriptor = 1U + MAX_BIT_NUMBER_BITS + // descriptor
                    m_firstElementBitSize + (m_numElements - 1) * deltaBitSize;
            const size_t unpackedBitSizeWithDescriptor = 1 + m_unpackedBitSize;
            if (packedBitSizeWithDescriptor >= unpackedBitSizeWithDescriptor)
            {
                resetFlag(IS_PACKED_FLAG);
            }
        }
    }
 
    size_t bitSizeOfDescriptor() const
    {
        if (isFlagSet(IS_PACKED_FLAG))
        {
            return 1 + MAX_BIT_NUMBER_BITS;
        }
        else
        {
            return 1;
        }
    }
 
    template <typename ARRAY_TRAITS,
            typename std::enable_if<has_owner_type<ARRAY_TRAITS>::value, int>::type = 0>
    static size_t bitSizeOfUnpacked(
            const typename ARRAY_TRAITS::OwnerType& owner, typename ARRAY_TRAITS::ElementType element)
    {
        return ARRAY_TRAITS::bitSizeOf(owner, element);
    }
 
    template <typename ARRAY_TRAITS,
            typename std::enable_if<!has_owner_type<ARRAY_TRAITS>::value, int>::type = 0>
    static size_t bitSizeOfUnpacked(const DummyOwner&, typename ARRAY_TRAITS::ElementType element)
    {
        return ARRAY_TRAITS::bitSizeOf(element);
    }
 
    void readDescriptor(BitStreamReader& in)
    {
        if (in.readBool())
        {
            setFlag(IS_PACKED_FLAG);
            m_maxBitNumber = static_cast<uint8_t>(in.readBits(MAX_BIT_NUMBER_BITS));
        }
        else
        {
            resetFlag(IS_PACKED_FLAG);
        }
    }
 
    template <typename ARRAY_TRAITS,
            typename std::enable_if<has_owner_type<ARRAY_TRAITS>::value, int>::type = 0>
    typename ARRAY_TRAITS::ElementType readUnpacked(
            const typename ARRAY_TRAITS::OwnerType& owner, BitStreamReader& in)
    {
        const auto element = ARRAY_TRAITS::read(owner, in);
        m_previousElement = static_cast<uint64_t>(element);
        return element;
    }
 
    template <typename ARRAY_TRAITS,
            typename std::enable_if<!has_owner_type<ARRAY_TRAITS>::value, int>::type = 0>
    typename ARRAY_TRAITS::ElementType readUnpacked(const DummyOwner&, BitStreamReader& in)
    {
        const auto element = ARRAY_TRAITS::read(in);
        m_previousElement = static_cast<uint64_t>(element);
        return element;
    }
 
    void writeDescriptor(BitStreamWriter& out) const
    {
        const bool isPacked = isFlagSet(IS_PACKED_FLAG);
        out.writeBool(isPacked);
        if (isPacked)
        {
            out.writeBits(m_maxBitNumber, MAX_BIT_NUMBER_BITS);
        }
    }
 
    template <typename ARRAY_TRAITS,
            typename std::enable_if<has_owner_type<ARRAY_TRAITS>::value, int>::type = 0>
    void writeUnpacked(const typename ARRAY_TRAITS::OwnerType& owner, BitStreamWriter& out,
            typename ARRAY_TRAITS::ElementType element)
    {
        m_previousElement = static_cast<uint64_t>(element);
        ARRAY_TRAITS::write(owner, out, element);
    }
 
    template <typename ARRAY_TRAITS,
            typename std::enable_if<!has_owner_type<ARRAY_TRAITS>::value, int>::type = 0>
    void writeUnpacked(const DummyOwner&, BitStreamWriter& out, typename ARRAY_TRAITS::ElementType element)
    {
        m_previousElement = static_cast<uint64_t>(element);
        ARRAY_TRAITS::write(out, element);
    }
 
    void setFlag(uint8_t flagMask)
    {
        m_flags |= flagMask;
    }
 
    void resetFlag(uint8_t flagMask)
    {
        m_flags &= static_cast<uint8_t>(~flagMask);
    }
 
    bool isFlagSet(uint8_t flagMask) const
    {
        return ((m_flags & flagMask) != 0);
    }
 
    static const uint8_t MAX_BIT_NUMBER_BITS = 6;
    static const uint8_t MAX_BIT_NUMBER_LIMIT = 62;
 
    static const uint8_t INIT_STARTED_FLAG = 0x01;
    static const uint8_t IS_PACKED_FLAG = 0x02;
    static const uint8_t PROCESSING_STARTED_FLAG = 0x04;
 
    uint64_t m_previousElement = 0;
    uint8_t m_maxBitNumber = 0;
    uint8_t m_flags = 0x00;
 
    uint8_t m_firstElementBitSize = 0;
    uint32_t m_numElements = 0;
    size_t m_unpackedBitSize = 0;
};
 
} // namespace zserio
 
#endif // ZSERIO_DELTA_CONTEXT_H_INC